Getting raw HTML / adding Javascript to WebView pages

I have a mobile flow for registration that I want to use in my app, and I do not have control of the source -- I am looking for a way to grab a few pieces of data when the user finishes registering, (confirmation number, etc.) that will be sent to my app (hopefully via Android's addJavascriptInterface)

I am certain on one thing - the id of the element I need. The flow could change, and is already a few pages long, So I'm looking for a general solution. The basic Idea I'm hoping for is this:

Inject a JavaScript snippet to each page during shouldOverrideUrlLoading, which will automatically call my JavaScript Interface and check for the value of the field with the id I'm looking for. (or just return the entire HTML, and I'll do it in Java)

view.setWebViewClient(new WebViewClient() {
             @Override
            public boolean shouldOverrideU开发者_开发知识库rlLoading(WebView view, String url) {
                 //inject javascript here to get value
                 return false;
            }});

I've seen tutorials on using addJavascriptInterface, but they all seem to assume some control or understanding of the 'single' page that will be navigated to. Since I have a potentially lengthy flow (3+ pages) and no control over the source, I am interested in hearing any suggestions.

Check URL of the current page and if it's the right one insert JavaScript in the curent page. With the help of Java-JS binding you could get some data out. See my previous answer: Determine element of url from webview shouldOverRideUrlLoading